Compressed air which leaves a compressor is always wet and consequently will cause many inconveniences. For that reason compressed air dryers have been developed.
The first technology (about 1920) made use of the adsorption technology resulting in dried compressed air having a dew point of -20.degree. C. or lower, for instance -30.degree. C. or even -40.degree. C. Typical adsorbents were porous materials having a high internal surface area on which the water vapour was absorbed. In this respect it is notable that the water absorbed by the adsorbents can be removed in a regeneration step by passing expanded compressed air over the adsorbent or directing a flow of very hot (.+-.200.degree. C.) air over the adsorbent.
The adsorption drying of compressed air was carried out by two main types of systems, based on the two ways of regenerating the saturated adsorbent i.e. with heat and without heat. Hot air drying systems used electrical heat or steam for regenerating the water saturated adsorbent. In this respect there are two types of heating, i.e.
the "Blower type" system comprising a dryer having an external heater and a ventilator, and PA1 the "Internal Heater" system comprising a dryer based on an internal heater and purge air for removing the desorbed water. PA1 introducing compressed air into one end of a hollow fiber membrane unit, which membrane is selective for water vapour permeation; PA1 introducing dry purge air, coming from an atmospheric adsorption dryer, into the permeate- or shell-side of the hollow fiber membrane unit; PA1 introducing dry purge air, coming from the same atmospheric adsorption dryer as above, into the regeneration inlet of said atmospheric adsorption dryer; and PA1 collecting the dried compressed air at the other end of the hollow fiber membrane unit as well as discharging the applied purge air.
More widely used are the so-called heatless systems, according to which a part of the dried air compressed is expanded and used for regeneration of the adsorbent. More particularly the principle of the heatless system can be explained as follows.
The drier is built with two pressure vessels, filled with an adsorbent (aluminium oxide or silica gel). The compressed air is led through one of the adsorber vessels to be dried. After drying, a part of the dried compressed air, in general 15%, is expanded to atmospheric conditions and then led through the second adsorber vessel in opposite flow direction. This dried expanded air will remove the absorbed water from the absorbent present in the second vessel. After every 2-5 minutes, the so-called cycle time, the flow of compressed air is changed from the first vessel to the second vessel wherein the compressed air is dried by the regenerated absorbent whereas the first vessel is now brought to atmospheric conditions and generated in the way, described above for the second vessel. Through interconnecting piping and valves the change over every 2-5 minutes is realized.
Compared to the above-described heat generated airdriers the heatless system has the advantage of being simple in operation. However, a disadvantage of the heatless system is found in the high operation costs, due to the use of 15% of the expensive compressed air for regeneration purposes of the adsorbents. Moreover each changeover of a pressure vessel under pressure to atmospheric pressure by blowing off compressed air incurs in a further loss resulting in a total loss of about 17% of the compressed air.
A second technology was developed in the early sixties. This technology uses a refrigeration system for cooling the compressed air to 3-5.degree. C. Thereafter the water was removed by means of a water separator. The obtained cold air is reheated by means of heat exchange with the warm incoming compressed air. However, these second technology systems do result in compressed air having a dew point of +3.degree. C., which is considered disadvantageous on account of the freezing of the water in such dried compressed air.
A third technology, i.e. the membrane technology, came up in the 1990's. The principle of this third technology may be elucidated as follows.
Wet compressed air is fed into a bundle of hollow fibers and is passed down the inside of said hollow fibers. The hollow fibers are present in a module. The water vapour passes the selective membrane wall much faster than air and the permeated water vapour is collected in the module shell. The driving force for this process is provided by the compressed air which flows on the permeate or shell side of the membrane wall. Further the permeated water vapour is preferably removed by means of a purge gas, for instance expanded compressed air, which is fed into the shell-side of the hollow fiber membrane module. This type of high selective membrane dryer may reach dew points of -60.degree. C. depending on the compressed air loss which is normally between 15% and 40% of the incoming compressed air. Further it is also possible to apply a vacuum pump at the shell-side of the hollow fiber membrane module for exhausting the water vapour, permeated through the hollow fibers. However, such an embodiment requires the use of an extra exhaust system i.e. a vacuum pump.
Summarizing it is stated that the above-discussed adsorption technology and the current membrane technology are the only technologies which can reach dew points lower than the freezing point, i.e. below 0.degree. C. Such a low dew point is a requisite if compressed air has to be used in outside environment application. In case the outside temperature may reach values below 0.degree. C. the water in compressed air having a dew point above 0.degree. C. will freeze and may cause corrosion and damage to the installation. However, both above technologies have the disadvantage of a high energy consumption, to be attributed to the loss of compressed air or the use of large electric heaters for heating the air for regenerating the adsorbents.
In view of the above it is referred to Derwent Publications Ltd., London, GB; Class J01, AN 91-225721. According to said reference water vapour containing gas is dehumidified by means of a gas-separator comprising a gas separation membrane. More in particular the water vapour containing gas is supplied in compressed state on one side of said membrane and simultaneously a compressed air in expanded state is supplied counter-currently as purge gas on the other side of said membrane. However, the use of compressed air in expanded state as a purge gas is rather expensive and therefore uneconomical.
Further, U.S. Pat. No. 5,240,472 relates to a process for the removal of moisture from a moisture laden gas stream, utilizing both a membrane dryer and an adsorption dryer. According to this known process the membrane dryer, comprising a very specific water vapour permeating membrane, is used as a "predryer", whereas the adsorption dryer is used as the "final dryer". The purge gas for the membrane dryer unit is a nearly moisture-free waste gas, typically waste nitrogen from a cryogenic air separation plant (column 5, lines 27-29).